A mitochondrial mutation A4401G is involved in the pathogenesis of left ventricular hypertrophy in Chinese hypertensives

Reduction of Drosophila Mitochondrial RNase P in Skeletal and Heart Muscle Causes Muscle Degeneration, Cardiomyopathy, and Heart Arrhythmia

In this study, we examine the cause and progression of mitochondrial diseases linked to the loss of mtRNase P, a three-protein complex responsible for processing and cleaving mitochondrial transfer RNAs (tRNA) from their nascent transcripts. When mtRNase P function is missing, mature mitochondrial tRNA levels are decreased, resulting in mitochondrial dysfunction. mtRNase P is composed of Mitochondrial RNase P Protein (MRPP) 1, 2, and 3. MRPP1 and 2 have their own enzymatic activity separate from MRPP3, which is the endonuclease responsible for cleaving tRNA. Human mutations in all subunits cause mitochondrial disease. The loss of mitochondrial function can cause devastating, often multisystemic failures. When mitochondria do not provide enough energy and metabolites, the result can be skeletal muscle weakness, cardiomyopathy, and heart arrhythmias. These symptoms are complex and often difficult to interpret, making disease models useful for diagnosing disease onset and progression. Previously, we identified Drosophila orthologs of each mtRNase P subunit (Roswell/MRPP1, Scully/MRPP2, Mulder/MRPP3) and found that the loss of each subunit causes lethality and decreased mitochondrial tRNA processing in vivo. Here, we use Drosophila to model mtRNase P mitochondrial diseases by reducing the level of each subunit in skeletal and heart muscle using tissue-specific RNAi knockdown. We find that mtRNase P reduction in skeletal muscle decreases adult eclosion and causes reduced muscle mass and function. Adult flies exhibit significant age-progressive locomotor defects. Cardiac-specific mtRNase P knockdowns reduce fly lifespan for Roswell and Scully, but not Mulder. Using intravital imaging, we find that adult hearts have impaired contractility and exhibit substantial arrhythmia. This occurs for roswell and mulder knockdowns, but with little effect for scully. The phenotypes shown here are similar to those exhibited by patients with mitochondrial disease, including disease caused by mutations in MRPP1 and 2. These findings also suggest that skeletal and cardiac deficiencies induced by mtRNase P loss are differentially affected by the three subunits. These differences could have implications for disease progression in skeletal and heart muscle and shed light on how the enzyme complex functions in different tissues.

Co-occurrence of m.15992A>G and m.15077G>A Is Associated With a High Penetrance of Maternally Inherited Hypertension in a Chinese Pedigree

BACKGROUND

Mitochondrial DNA (mtDNA) pathogenic variants have been identified to be associated with maternally inherited essential hypertension (MIEH). However, the distinctive clinical features and molecular pathogenesis of MIEH are not fully understood.

METHODS

In this study, we collected a Chinese MIEH family with extraordinary higher penetrance of essential hypertension (88.89%) and early ages of onset (31-40 years old), and performed clinical and genetic characterization for this family. The complete mitochondrial genome of the proband was sequenced and analyzed.

RESULTS

The maternally related members in this family were presented with severe increased blood pressure, left ventricular remodeling, and metabolic abnormalities. Through sequencing the entire mtDNA of the proband and performing systematic analysis of the mtDNA variants with a phylogenic approach, we identified a potentially pathogenic tRNA variant (m.15992A>G in the MT-TP gene) that may account for the MIEH in this family. One nonsynonymous variant (m.15077G>A in the MT-CYB gene) was identified to play a synergistic role with m.15992A>G to cause a high penetrance of MIEH.

CONCLUSIONS

Our results, together with previous findings, have indicated that tRNA pathogenic variants in the mtDNA could act important roles in the pathogenesis of MIEH through reducing mitochondrial translation and disturbing mitochondrial function.

Disease-associated mutations in mitochondrial precursor tRNAs affect binding, m1R9 methylation, and tRNA processing by mtRNase P

Mitochondrial diseases linked to mutations in mitochondrial (mt) tRNA sequences are common. However, the contributions of these tRNA mutations to the development of diseases is mostly unknown. Mutations may affect interactions with (mt)tRNA maturation enzymes or protein synthesis machinery leading to mitochondrial dysfunction. In human mitochondria, in most cases the first step of tRNA processing is the removal of the 5' leader of precursor tRNAs (pre-tRNA) catalyzed by the three-component enzyme, mtRNase P. Additionally, one component of mtRNase P, mitochondrial RNase P protein 1 (MRPP1), catalyzes methylation of the R9 base in pre-tRNAs. Despite the central role of 5' end processing in mitochondrial tRNA maturation, the link between mtRNase P and diseases is mostly unexplored. Here, we investigate how 11 different human disease-linked mutations in (mt)pre-tRNA^Ile, (mt)pre-tRNA^Leu(UUR), and (mt)pre-tRNA^Met affect the activities of mtRNase P. We find that several mutations weaken the pre-tRNA binding affinity (K_D s are approximately two- to sixfold higher than that of wild-type), while the majority of mutations decrease 5' end processing and methylation activity catalyzed by mtRNase P (up to ∼55% and 90% reduction, respectively). Furthermore, all of the investigated mutations in (mt)pre-tRNA^Leu(UUR) alter the tRNA fold which contributes to the partial loss of function of mtRNase P. Overall, these results reveal an etiological link between early steps of (mt)tRNA-substrate processing and mitochondrial disease.

Associations of mitochondrial DNA 3777-4679 region mutations with maternally inherited essential hypertensive subjects in China

Background

Nuclear genome or family mitochondrial screening system has become the hot focus of studies into essential hypertension. The role of mitochondrial DNA (mtDNA) in sporadic Chinese patients with hypertension has not been fully understood. The study was to evaluate the associations of mtDNA mutations with maternally inherited essential hypertensive subjects in China.

Methods

From June 2009 to June 2016, a total of 800 gender-matched Chinese patients with maternally inherited essential hypertension (MIEH) and control group were 1:1 enrolled in this case-control study. Genomic DNA was extracted from each person’s peripheral blood cells. The main mtDNA locations for MIEH were screened with oligodeoxynucleotides 3777-4679 bp, analyzed and compared with the updated consensus Cambridge Sequence. Pathogenic mtDNA mutations were identified from the mitochondrial map.

Results

MIEH subjects presented significantly higher values than those of control group in abdominal circumference (AC), waist circumference (WC), body mass index (BMI), fasting blood glucose (FBG), triglyceride (TG), low-density lipoprotein cholesterol (LDL) and renal function (P < 0.05). MIEH subjects carried more amino acid changes and coding sequence variants (P < 0.01) than control group. The allele frequencies of the eight single nucleotide polymorphisms (SNPs) were significantly different between the two groups, including m.3970 C > T, m.4048G > A, m.4071C > T, m.4086C > T, m. 4164A > G and m.4248 T > C in ND1 gene, and m.4386 T > C and m.4394C > T in tRNA^Gln gene(P < 0.001). Fifty-five homoplasmic or heteroplasmic mutations were detected in 5 genes: ND1, tRNA^Ile, tRNA^Met, tRNA^Gln and ND2 gene. The ND1 gene was the main mutation site, where the most mtDNA mutation was m.3970 C > T.

Conclusions

The mtDNA mutations were involved in the process of MIEH. We identified mitochondrial genetic characteristics in MIEH patients in China. The present research serves as a solid foundation for further detailed research on the association between MIEH and mitochondrial dysfunction, and their causal relationship in Chinese and other populations with a similar lifestyle.

A high prevalence of arterial hypertension in patients with mitochondrial diseases

The prevalence of arterial hypertension in mitochondrial diseases remains unknown. Between January 2000 and May 2014, we retrospectively included patients with genetically proven mitochondrial diseases. We recorded clinical, genetic and cardiac exploration data, including the measure of arterial pressure. Among the 260 patients included in the study (mean age = 44 ± 15 years, women = 158), 108 (41.5%) presented with arterial hypertension. The prevalence of hypertension by sex and age was higher than that observed in the general population for all groups. The prevalence of hypertension was significantly higher in patients with MELAS (mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes) mutations (66%) and MERRF (myoclonus, epilepsy, ataxia with ragged ref fibres) mutations (61%). In patients with MELAS mutation, the presence of hypertension was significantly associated with age and mutation rate in the blood (odds ratio = 1.12; P = .02) in multivariate analysis. The prevalence of hypertension was more important in patients having a mitochondrial disease. The increased risk was more important in patient with MELAS or MERRF and depended on the rate of heteroplasmy.

Integrated approach for data acquisition, visualization and processing of analog polarographic systems for bioenergetics studies

Bioenergetic function is characterized with assays obtained by polarographic systems. Analog systems without data acquisition, visualization, and processing tools are used but require cumbersome operations to derive respiration rate and ADP to oxygen stoichiometry of oxidative phosphorylation (ADP/O ratio). The analog signal of a polarograhic system (YSI-5300) was digitized and a graphical user interface (GUI) was developed in MATLAB to integrate visualization, recording, calibration and processing of bioenergetic data. With the GUI, the signal is continuously visualized during the experiment and respiratory rates and ADP/O ratios can be determined. The integrated system was tested to evaluate bioenergetic function of subpopulations of mitochondria isolated from rat skeletal muscle (n = 10). Signal processing was applied to denoise data recorded at the sampling rate of 1000Hz, and maximize data decimation for computational applications. The error in calculating the bioenergetic outputs using decimated data is negligible when data are denoised. The estimate of respiration rate, ADP/O ratio and RCR obtained with denoised data at sampling rate as low as 5Hz was similar to that obtained with raw data recorded at sampling rate of 1000Hz. In summary, the integrated tools of the GUI overcome the limitations of data processing, accuracy, and utilization of analog polarographic systems.

Mitochondrial DNA 7908-8816 region mutations in maternally inherited essential hypertensive subjects in China

Background

Nuclear genes or family-based mitochondrial screening have been the focus of genetic studies into essential hypertension. Studies into the role of mitochondria in sporadic Chinese hypertensives are lacking. The objective of the study was to explore the relationship between mitochondrial DNA (mtDNA) variations and the development of maternally inherited essential hypertension (MIEH) in China.

Methods

Yangzhou residents who were outpatients or in-patients at the Department of Cardiology in Northern Jiangsu People’s Hospital (Jiangsu, China) from June 2009 to June 2015 were recruited in a 1:1 case control study of 600 gender-matched Chinese MIEH subjects and controls. Genomic DNA was isolated from whole blood cells. The most likely sites for hypertension were screened using oligodeoxynucleotides at positions 7908–8816, purified and subsequently analyzed by direct sequencing according to the revised consensus Cambridge sequence. The frequency, density, type and conservative evolution of mtDNA variations were comprehensively analyzed.

Results

We found a statistical difference between the two groups for body mass index, waist circumference, abdominal circumference, triglyceride, low-density lipoprotein cholesterol, fasting blood glucose, uric acid, creatinine and blood urea nitrogen (P < 0.05). More amino-acid changes and RNA variants were found in MIEH subjects than the controls (P < 0.01). The detection system simultaneously identified 40 different heteroplasmic or homoplasmic mutations in 4 genes: COXII, tRNA^Lys, ATP8 and ATP 6. The mtDNA variations were mainly distributed in regions of ATP6 binding sites, and the site of highest mutation frequency was m. 8414C > T. Three changes in single bases (C8414T in ATP8, A8701G in ATP6 and G8584A in ATP6) were significantly different in the MIEH patients and the controls (P < 0.001). The m.8273_8281del mutation was identified from 59 MIEH patients.

Conclusions

Our results indicate that novel mtDNA mutations may be involved in the pathological process of MIEH, and mitochondrial genetic characteristics were identified in MIEH individuals.

Electronic supplementary material

The online version of this article (10.1186/s12920-018-0408-0) contains supplementary material, which is available to authorized users.

Molecular characterization of mitochondrial transferRNAGln and transferRNAMet A4401G mutations in a Chinese family with hypertension

Mutations in mitochondrial (mt)transfer (t)RNA (mt‑tRNA) have been reported to serve important roles in hypertension. To determine the underlying molecular mechanisms of mt‑tRNA mutations in hypertension, the present study screened for mt‑tRNA mutations in a Chinese family with a high incidence of essential hypertension. Sequence analysis of the mt‑tRNA genes in this family revealed the presence of an A4401G mutation in the glycine‑and methionine‑tRNA genes, and a G5821A mutation in the cysteine‑tRNA (tRNACys) gene. The G5821A mutation was located at a position conserved in various species, and disrupted G6‑C67 base‑pairing. It was hypothesized that the G5821A mutation may decrease the baseline expression levels of tRNACys, and consequently result in failure of tRNA metabolism. The A4401G mutation was reported to cause the mitochondrial dysfunction responsible for hypertension. Thus, the combination of G5821A and A4401G mutations may contribute to the high incidence of hypertension in this family. Mt‑tRNA mutations may serve as potential biomarkers for hypertension.

Mutational analysis of mitochondrial DNA in Brugada syndrome

BACKGROUND

Brugada syndrome (BrS) is a primary electrical disease associated with an increased risk of sudden cardiac death due to ventricular fibrillation. This pathology has nuclear heterogeneous genetic origins, and at present, molecular diagnostic tests on nuclear DNA cover only 30% of BrS patients. The aim of this study was to assess the possible involvement of mitochondrial (mt) DNA variants in BrS since their etiological role in several cardiomyopathies has already been described.

METHODS AND RESULTS

The whole mt genome of BrS patients was sequenced and analyzed. A specific mtDNA mutation responsible for BrS can be excluded, but BrS patient d-loop was found to be more polymorphic than that of control cases (P=0.003). Moreover, there appears to be an association between patients with the highest number of variants (n>20) and four mt Single Nucleotide Polymorphism (SNPs) (T4216C, A11251G, C15452A, T16126C) and the most severe BrS phenotype (P=0.002).

CONCLUSIONS

The high substitution rate found in BrS patient mtDNA is unlikely to be the primary cause of the disease, but it could represent an important cofactor in the manifestation of the BrS phenotype. Evidence suggesting that a specific mtDNA allelic combination and a high number of mtDNA SNPs may be associated with more severe cases of BrS represents the starting point for further cohort studies aiming to test whether this mt genetic condition could be a genetic modulator of the BrS clinical phenotype.

Mitochondrial transcript maturation and its disorders

Mitochondrial respiratory chain deficiencies exhibit a wide spectrum of clinical presentations owing to defective mitochondrial energy production through oxidative phosphorylation. These defects can be caused by either mutations in the mitochondrial DNA (mtDNA) or mutations in nuclear genes coding for mitochondrially-targeted proteins. The underlying pathomechanisms can affect numerous pathways involved in mitochondrial biology including expression of mtDNA-encoded genes. Expression of the mitochondrial genes is extensively regulated at the post-transcriptional stage and entails nucleolytic cleavage of precursor RNAs, RNA nucleotide modifications, RNA polyadenylation, RNA quality and stability control. These processes ensure proper mitochondrial RNA (mtRNA) function, and are regulated by dedicated, nuclear-encoded enzymes. Recent growing evidence suggests that mutations in these nuclear genes, leading to incorrect maturation of RNAs, are a cause of human mitochondrial disease. Additionally, mutations in mtDNA-encoded genes may also affect RNA maturation and are frequently associated with human disease. We review the current knowledge on a subset of nuclear-encoded genes coding for proteins involved in mitochondrial RNA maturation, for which genetic variants impacting upon mitochondrial pathophysiology have been reported. Also, primary pathological mtDNA mutations with recognised effects upon RNA processing are described.

Impaired 3',5'-cyclic adenosine monophosphate-mediated signaling in immediate early responsive gene X-1-deficient vascular smooth muscle cells

Gene-targeted deletion of the immediate early responsive gene X-1 (IEX-1) results in a significant increase in systemic arterial blood pressure, but the underlying mechanism is not understood. Studies of arterial reactivity in isolated aortas revealed normal endothelium-dependent and -independent vasorelaxation and vasoconstriction but reduced cAMP-dependent vasorelaxation in the absence of IEX-1. This defect in cAMP signaling was also evident in endothelium-denuded aortic rings, consistent with the enhancement of mitochondrial O2·- production only in IEX-1-deficient vascular smooth muscle cells, not in endothelial cells. Excessive production of reactive oxygen species at mitochondria augmented the expression of Gα(i2), suppressing cAMP production in vascular smooth muscle cells. The role of mitochondrial reactive oxygen species in the upregulation of Gα(i2) leading to the development of hypertension was supported by the ability of antioxidant or pertussis toxin to restore the cAMP-dependent vasorelaxation to a normal level and reverse established hypertension in IEX-1 homozygous knockout mice. Our results suggest that hypertension in IEX-1 knockout mice may arise primarily from impaired cAMP signaling induced by overproduction of mitochondrial reactive oxygen species in vascular smooth muscle cells and demonstrate a causal relationship between mitochondrial dysfunction and cAMP-dependent vasorelaxation.

Genetic variants in mitochondrial tRNA genes are associated with essential hypertension in a Chinese Han population

BACKGROUND

Of multiple factors contributing to essential hypertension, mitochondrial variants exhibited the trends for serving as molecular and genetic markers for the disease in last five years. However, previous studies focused on African-American or Caucasian pedigrees, knowledge of mitochondrial tRNA genes and population-based Chinese hypertensives were limited.

METHODS

We performed sequence analysis in tRNA genes, hot spots for cardiovascular diseases, in 270 Chinese Han essential hypertensives and 270 controls. Lymphoblastoid cell lines were immortalized by transformation with the Epstein-Barr virus. Rates of oxygen consumption in intact cells were determined with a YSI 5300 oxygraph (Yellow Springs Instruments) on samples, harboring variants in tRNA genes.

RESULTS

There were 26 variants in tRNA genes that were found in hypertensives and these variants were not in controls. Functional analysis found that these variants may lead to deficiencies in tRNA 3' end metabolism and/or impairment of critical subunits of the respiratory chain. Most importantly, the oxygen consumption rate in cells harboring variants T4454C (P=0.0010) and A4263G (P=0.0001) decreased as compared to the average level of control cell lines.

CONCLUSIONS

Variants located in mitochondrial tRNA genes may have biologic plausibility to implicate in the pathogenesis of Chinese essential hypertension.